The organic light-emitting device was first reported on its high-luminance emission by C. W. Tang et al of Kodak in 1987 (see, Appl. Phys. Lett., Vol. 51, page 913 (1987)). Since then, an abrupt progress has been proceeding in the development of materials and improvement of device structures and in recent years, the organic light-emitting device is actually used in a display for car audios or cellular phones. In order to more expand the use of this organic EL, development of materials for improving the emitting efficiency or durability or development of full color displays are being aggressively made at present. Particularly, on considering the use wide-spreading to the medium- or large-size panel or illumination, the high luminance must be more intensified by improving the emitting efficiency. However, the currently known light-emitting materials use light emission from the excited singlet state, namely, fluorescence, and according to Monthly Display, “Organic EL Display”, extra number, page 58 (October, 1988), the generation ratio of the excited singlet state to the excited triplet state upon electric excitation is 1:3. Therefore, the internal quantum efficiency in the fluorescence emission in an organic EL device has been acknowledged to have an upper limit of 25%.
On the other hand, M. A. Baldo et al. have reported that an external quantum efficiency of 7.5% (assuming that the external couple out efficiency is 20%, the internal quantum efficiency is 37.5%) can be obtained by using an iridium complex capable of emitting phosphorescence from the excited triplet state and thus, the conventionally acknowledged upper limit of 25% can be surpassed (see, Appl. Phys. Lett., Vol. 75, page 4 (1999)). However, such a material that is capable of stably emitting phosphorescence at room temperature like the iridium complex used there is very rare so that freedom in selecting a material is narrow, and on practical use, the material must be disadvantageously doped into a specific host compound. As a result, great difficulties are encountered in selecting a material for satisfying the specification necessary for displays.
Furthermore, the same M. A. Baldo et al. have reported that relatively good emitting efficiency can be obtained by using an iridium complex as a sensitizer, transferring the energy from the excited triplet state of this complex to the excited singlet state of a fluorescent dye, and finally emitting fluorescence from the excited singlet state of the fluorescent dye (see, Nature, Vol. 403, page 750 (2000)). This method is advantageous in that a light-emitting material well matching the purpose can be selected from a large number of fluorescent dyes. However, this method has a serious problem that it involves energy transfer from the excited triplet state of a sensitizer to the excited singlet state of a fluorescent dye, which is a spin-forbidding process, so that the emission quantum efficiency is low in principle.
As such, existing light-emitting materials for use in an organic light-emitting device cannot succeed in surpassing the conventionally acknowledged marginal value of 25% in the internal quantum efficiency and being applicable to all emission colors considered necessary for a display. That is, a material system other than the transient metal complex such as iridium that emits phosphorescence at room temperature and provides freedom of selecting an emission color has been demanded. A material having high emitting efficiency is demanded also from the standpoint of improving the durability of the device because such a material causes little energy loss and the device can be prevented from heat generation. An object of the present invention is to solve those problems in conventional techniques and provide a high-luminance organic light-emitting device having durability and a light-emitting material for use in the device.